CARIUUEAN SEA ECOS\'STHI ASSESSMENT Scagrass communities serve as habitats for a wide range of' organisms. They provide food for species such as parrot fish, surgeonlish. queen conch, sea urchins. and green turtles. The seagrass leaves carry epiphytic algae" and animals, which arc grazed by invertebrates and !ish. The scagrass blades enhance sedimentation and reduce erosion by slowing down waves and currents, while the roots and rhizomes" bind and stabilize the sediment surface. Seagrass beds arc very important in the marine !bod chain as a resull of' the high rate at which they convert carbon dioxide dissolved in the water into organic muller, through the process of photosynthesis (high net productivity). This rate, approximately I kg of carbon lor each square metre in the course of a year (I kg C m 'year ') is signilicant because about half' of this material is exported as detritus, which contributes !bod to o!Tshore ecosystems. Seagrass habitats act as a nursery lor the young of' many commercial species of' !ish. crustaceans, and molluscs, based carnivores venture olTinto nearby scagrass while beds in search of !bod. The wide variety or epiphytes which live in the seagrasses become the !bod of many bottom-dwelling !ish species which feed o!T detritus. Organisms in seagrass beds with calcium-based external skeletons (lor example, molluscs, echinoderms, crustaceans, calcareous algae. and some protozoa) also help to lbnn beach sand. Threats to scagrass beds in the Caribbean include their removal from shallow water to "improve" bathing beaches; dredging to allow access to shipping or to lay cables. pipes, and other submarine structures; burying by sediment from nearby dredging and tilling activities; and pollution li·mn mnricnts such as nitrogen which causes excessive growth of epiphytes. Nutrient pollution can also overstimulate the growth of the seagrass itself: leading to dil1icult decisions on whether to clear beds which expand into previously unsettled sandy areas. In summary, scagrass colonies arc undervalued lor the contribution they make to key services of the Caribbean Sea ecosystem, including !ish cries (directly) and tourism (indirectly) through the production of sand, protection from wave action, and nurturing of wildlife important lor ceo-tourism). An cpiphyh! h; 1.111 organism which gruws naturally on another hut dues nul usl! it for nourishment. JJ A rhizome is a plant stem whil:h grows bt.!nt!alh the soil or scdimt.!nl surfhcc. 13 2. 7.3 Com/ Coral reefs arc among the most productiw tropical marine ecosystems and have the highest biodiversity (MA 2005b). Corals arc sessile (immobile) organisms whose bodies arc in the form of a small polyp, usually less than l em in diameter. corals arc colonial, occurring as sheets of many thousands of polyps over a calciumcarbonate skeleton. Corals arc found in all of the oceans, This but it is only in the tropics that they fonn is done with the help of symbiotic singlc-ccllcd algae contained in their tissues. The algae usc sunlight to catTy out photosynthesis and provide organic nourishment lor the corals, and also help to deposil the skeleton. The accumulated skeletons of many generations of comls, cemented together with other carbonate sediments, lonn the rcc[ The living corals, and the spurs and canyons within the reel: give the three-dimensional structure which provides habitats lor so many species. The typical structure of a Caribbean fringing reef includes a lagoon tens to hundreds of metres wide, a shallow platlbnn or reef llat, a de lined reef crest, and a more or less steeply sloping lore-reef on which spurs may develop. Seven per cent of the world's coral reels arc located in the \Vider Caribbean (Burke and Maidens 2004). They include the Meso-American reel: the largest com! system in the Northern Hemisphere. stretching nearly a thousand kilometres from the northcm tip of the Yucatan peninsula in Mexico, along the coasts of Belize and Guatemala, to the north-cast shoreline or Honduras. Coral reels in the Caribbean Sea arc prolilic providers of ecosystem services, including food, protection from storms. recreational value and therefore tourism income, and medicinal products. It is estimated that the potential yields for fisheries from coral reefs amount to 10 t km 1 year 1, which could provide up to 6% of global lisherics if properly managed (Burke and Maidens 2004). Commercially valuable species fished on coral reels include snappers (Lutjanidac), groupers (Scrranidae), and jacks (Carangidac), while less valuable species include parrot !ish (Sparidac) and surgeon fish (Acanthuridae). Important shclllishcrics include those for conch (a large marine gastropod mollusc) and lobster. Harvesting of other reef resources includes live ornamental fish lor the aquarium trade; collection of coral skeletons and shells of other creatures lor jewellery and other ornaments; mining of reef rock, coral heads, and coral sand for construction; and bioprospccting lor potential pharmaceuticals. Only a small fraction of the huge reef biodiversity has so far been tested for the presence of - 34-